1. Field of the Invention
The present invention relates to a direct-current (DC) power conversion circuit, and more particularly, to a DC power conversion circuit having self-auxiliary power and self-protection against a short or open load circuit.
2. Description of the Prior Art
In recent years, a light emitting diode (LED) has been widely applied to various IT and consumer electronic products, used in an indication light or display devices. Unlike a traditional incandescent light bulb, the LED is a luminescent device having advantages of low power consumption, long life, extremely short warm-up time and fast reaction. Moreover, the LED is fitly used in small-scale array devices because of its small size, high vibration tolerance and realization of mass production. According to the physical characteristic of the LED, an LED current varies with a forward bias across the LED by a exponential relationship, indicating that luminance of the LED is proportional to the current. That is, the larger the current the LED bears, the stronger light the LED emits. Thus, there is a need of a conversion circuit to control the forward bias in order for the LED to output steady light.
Please refer to FIG. 1, which is a schematic diagram of a DC/DC conversion circuit 10 according to the prior art. The DC/DC conversion circuit 10 includes a DC voltage source 100, a switch transistor 102, a resistor 104, a diode 106, an inductor 108, a control signal generator 110, a load circuit 112 and a linear regulator 114. The linear regulator 114 is commonly an IC or a simple bias circuit and used for regulating voltage provided by the DC voltage source 100 down to a proper voltage level, so as to provide working voltage for the control signal generator 110. The control signal generator 110 senses voltage or current across the two ends of the sensing resistor 104 and thereby generates a control signal Sc to control the switch transistor 102. With turn-on and turn-off of the switch transistor 102, the current outputted to the load circuit 112 can be controlled. Operations of the DC/DC conversion circuit 10 are simply described as follows. When the switch transistor 102 turns on, the diode 106 bears a reverse bias and thereby cuts off a loop L1 such that the DC voltage source 100 starts to charge the inductor 108. As soon as the switch transistor 102 turns off, the inductor 108 generates a reversed-polarity voltage. In other words, when the switch transistor 102 turns off, the diode 106 conducts the loop L1 so that the inductor 108 transfers electricity to the capacitor 110 and the load circuit 112.
In the prior art, the linear regulator 114 regulates voltage provided by the DC voltage source 100 for providing the control signal generator 110 with the working voltage. However, the linear regulator 114 of the DC/DC conversion circuit 10 usually has a poor efficiency of about 5%-10%, where the efficiency is a ratio of output voltage over input voltage of the linear regulator 114. For example, if the DC voltage source 100 provides a DC voltage of 170 Volts(V) in the DC/DC conversion circuit 10, the linear regulator 114 generally regulates the DC voltage down to 17V in a condition of a 10% efficiency. Assuming that a current across the linear regulator 114 is about 20 mA, the linear regulator 114 has power dissipation: (170−17)×20=3.06 Watt(W). As can be understood from the above, the prior art DC/DC conversion circuit 10 utilizes the linear regulator 114 having the poor efficiency, thereby resulting in large power dissipation. Therefore, the linear regulator 114 wastes energy and may be damaged due to overheat.